Radicals in organic chemistry Chain reactions

Posted by: @damiryagudin

Good evening!

Is it possible to call chain reactions autocatalytic or is it just a free radical reaction mechanism? What is the role of radicals in chain reactions?

Hi No autocatalytic, because free radicals are formed intermediate and are consumable components of the reaction Free radicals play a role in maintaining chain processes, determine the course, selectivity of the process, its speed, energy

Radical are a it's on little planet in the world of organic chemistry, that's for sure

@argentum If we assume that the radical formed during the chemical reaction, considered as an autocatalyst, will interact with the reagents. After their interaction, an intermediate radical is formed. As a result, the intermediate radical decays with the formation of the reaction product and the radical as an autocatalyst. Why not consider such a mechanism? This option has a place to be. In this case, it is not consumed. As an example, such a radical can be a peroxide radical, which has the most active reactivity. And I absolutely agree with you that there are cases when the radical is consumed as a result of its interaction with the reagent. After that, a new radical or new radicals are formed, which continue the chain reaction.

@chaoticgood I absolutely agree with you, considering this at the micro level.

@damiryagudin In this case, radicals are not stable compounds, in contrast to catalysts

@argentum Despite their instability, they play the role of an autocatalyst.

@argentum As is known, chain reactions at the initial stage proceed slowly with subsequent acceleration. This shows that along with the monomolecular reaction, an autocatalytic reaction takes place. At the chain initiation stage, free radicals are formed, which have a role as autocatalysts. If chain reactions did not occur at the beginning of the stage, then they would not be catalytic. In the subsequent stage of chain continuation, possibly free radicals play the role of an inductor more, which is consumed and without which a further continuation of the chain is not possible, unlike a catalyst. I think you'll agree with me. Again, all these are assumptions, so we are reasoning with you and have the right to do so 😀 

@damiryagudin The autocatalyst must leave the reaction and not be consumed, which is not the case in the case of radicals. By the way, in radical reactions, in the final, the chain breaks - the recombination of radicals, which also speaks not in favor of autocatalysis.

@damiryagudin I think the acceleration is due to an increase in the number of participating free radicals, which is comparable to the acceleration of reactions with an increase in the concentration of reactants

@argentum A characteristic feature of autocatalytic reactions is the presence of an induction period. The initial reaction period when the speed is very low. It is known that at the initial stage (chain initiation), the reaction proceeds, but slowly (radicals are formed from ordinary molecules). For example, liquid-phase oxidation of hydrocarbons, in which the formed radical R returns to the initiation stage at the chain continuation stage, thereby accelerating the oxidation of hydrocarbons. Such a radical can be called an autocatalyst of the process. For example, the radical ROO formed at the stage of chain continuation is consumed with the formation of other radicals, as well as at the stages of chain breakage and chain separation, radicals are consumed with the formation of new radicals or their breakage. What you talked about above.In this case, these radicals will be inductors, which initiates the transformation of some radicals into others. At what it is spent.

@damiryagudin I agree, but at the end of the reaction they do not come out unchanged

@argentum You understood correctly.

I think  there is no much reason to set such strict boundaries in terminology 

These are not strict boundaries, this is a theory that needs to be understood before studying a particular process.